FIG. 11 illustrates an example of a conventional superconducting magnet apparatus for use in MRI system. This example is a superconducting magnet apparatus of the horizontal magnetic field type. This superconducting magnet apparatus is composed of small-diameter main coils 13, 14, 15, 16, 17 and 18 and large-diameter shield coils 19 and 20 and is adapted to produce a horizontal (namely, Z-axis direction) magnetic field. In this example, the main coils 13 to 18 are placed to produce a magnetic field along the center axis 22 of a magnet, while the shield coils 19 and 20 are placed to shield magnetic field leakage to the surroundings thereof. With such a configuration of the magnet, a uniform magnetic field region 21 of magnetic homogeneity of about 10 ppm or less is formed in a magnetic field space. Magnetic resonance imaging is performed in this uniform magnetic field 21.
These coils are made by using superconducting wires, and thus are required to be cooled to a predetermined temperature (for example, liquid-helium temperature (namely, 4.2 K) in the case of alloy superconductors; and liquid-nitrogen temperature (namely, 77 K) in the case of oxide superconductors). The coils are, therefore, held in a cooling vessel consisting of a vacuum enclosure, a thermal shield and a coolant container (which contains liquid helium or the like). In the case of the example of FIG. 11, the main coils 13 to 18 and the shield coils 19 and 20 are placed in the coolant container 11 containing coolant 12, such as liquid helium, for superconductivity, and are supported by means of supporting elements (not shown). Further, the coolant container 11 is held in the vacuum enclosure 10.
Moreover, to keep each of the coils at a low temperature, the thermal shield is maintained at a constant temperature by using a refrigerator (not shown), or the evaporation of coolant 12 for superconductivity is reduced. Recently, the performance of refrigerators has been increased, so that the superconductor coils are sometimes, cooled directly by the refrigerator without using the coolant 12 for superconductivity.
However, in the case of the superconducting magnet apparatus illustrated in FIG. 11, an opening, in which a subject is accommodated and images of the subject are taken, is narrow and moreover, a measuring space is surrounded, so that subjects sometimes feel claustrophobic. Thus, occasionally, subjects refuse to enter the opening of the apparatus for examination. Furthermore, it is difficult for an operator to get access to a subject from the outside of the superconducting magnet apparatus.
FIG. 12 illustrates a second example of a conventional superconducting magnet apparatus for use in MRI system. This example is a superconducting magnet apparatus of the vertical magnetic field type. FIG. 12(a) is a schematic external view of this apparatus. FIG. 12(b) is a sectional view taken in the direction of an arrow A of FIG. 12(a). This example of the conventional superconducting magnet apparatus is disclosed in the U.S. Pat. No. 5,194,810. In this magnet, a magnetic field is produced by two sets of superconducting coils 23 and 23, which are placed vertically in a line in such a manner as to face each other. Further, iron shimming means 24 are provided inside each of the aforesaid superconducting coils 23 and 23 so as to obtain favorable magnetic homogeneity. Thereby, this magnet enhances the magnetic homogeneity of the uniform magnetic field region 21. Moreover, this magnet has a structure in which iron plates 25, 25 and iron yokes 26, 26, . . . also serving as return paths for magnetic fields generated by the upper and lower superconducting coils 23 and 23, are placed, each of the aforementioned iron plates 25 supporting the corresponding superconducting coil 23 and the corresponding shimming means 24 and the iron yokes 26 intervening between the iron plates 25, 25 to mechanically support the iron plates 25, 25.
In the case of this example of the conventional superconducting magnet apparatus, the uniform magnetic-field region 21 is opened in all directions, a subject can avoid feeling claustrophobic. Moreover, an operator can easily get access to the subject. Further, magnetic field leakage can be reduced because of the fact that the return path of a magnetic flux is composed of the aforementioned iron plates 25, 25 and the aforementioned iron yokes 26, 26, . . .
However, in the case of the superconducting magnet apparatus illustrated in FIG. 12, there are caused the problems that the iron plates 25 and the iron yokes 26 are used as above described, so that the entire magnet becomes heavy and that thus, when installing the superconducting magnet apparatus, an installation floor needs to be reinforced. Further, because the saturation magnetic flux density of iron is approximately 2 Tesla or so, there is a restraint on the magnet in that the magnetic field strength cannot be increased to a high value. Furthermore, there are possibilities that a magnetic field generated by a gradient magnetic field coil affects the magnetic field distribution due to the hysteresis characteristics of iron with respect to the magnetic field. This may hinder high-precision signal measurement
Inventors of the present invention filed another Japanese patent application (title of the invention: a superconducting magnet apparatus, applicant: Hitachi Medical Corporation, and application No.: 336023/1995), in which an invention (hereunder referred to as a "third example of the conventional superconducting magnet apparatus") is presented as having solved the problems of the aforementioned two examples of the conventional apparatus) on Nov. 30, 1995. Apparatus according to this invention is of the open vertical-magnetic-field type. The configuration of this apparatus is schematically illustrated in FIG. 13. FIG. 13(a) is a sectional view of this apparatus. FIG. 13(b) is an external view thereof. As shown in FIG. 13(a), two sets of superconducting coils, which are accommodated in vacuum enclosures 10A and 10B and coolant containers 11A and 11B, are spaced apart from each other by a predetermined distance in such a manner as to be coaxial with the center axis 22 of the magnet. Further, a uniform magnetic field region 21 is generated at the halfway position between the sets of the coils. The aforementioned superconducting coils are composed of: coils 31A and 31B (hereunder referred to as "main coils") for generating a primary magnetic field in a uniform magnetic region 21; coils 32A and 32B (hereunder referred to as "bucking coils") for canceling out an external magnetic field by generating a magnetic field, which is in opposite direction to the magnetic field produced by the main coils; and coils 33A, 34A, 35A, 33B, 34B and 35B (hereunder referred to as "regulating coils") for correcting the magnetic homogeneity of the uniform magnetic field region 21. The vacuum enclosures 10A and 10B are intervened and supported by supporting posts 36, 36. Characteristic aspect of this invention resides in that the magnetic leakage is reduced by canceling out the external magnetic field, which is produced by the main coils 31A and 31B, by the bucking coils 32A and 32B. In this case, iron is not used for reducing the leakage magnetic field. Consequently, there is not caused such a problem as in the second example of the conventional magnetic superconducting magnet apparatus.
However, in the case of this third example of the conventional superconducting magnet apparatus, the magnetic strength of the uniform magnetic field region 21 is also reduced because of the use of the bucking coils 32A and 32B. As a result, when increasing the magnetic strength of the uniform magnetic field region 21, magnetomotive forces required to the main coils 31A and 31B and the bucking coils 32A and 32B become huge. This results in rise of the manufacturing cost of the magnet apparatus. Further, the electromagnetic force applied to each of the superconducting coils increases according to the magnetomotive force. Therefore, strict structural conditions are imposed. Incidentally, generally, the shorter the distance between each of the bucking coils 32A and 32B and the corresponding one of the main coils 31A and 31B becomes, the more the magnetic fields generated by both of the bucking coil and the corresponding one of the main coils is canceled out. Consequently, the aforementioned problem becomes more serious.
In contrast, when increasing the distance between both of the bucking coil and the corresponding main coil, the size of each of the coolant containers 11A and 11B for containing these coils becomes large. This also causes a rise in the manufacturing cost of the apparatus. Further, the position of the uniform magnetic field region 21, into which a subject is inserted, from the floor face becomes high. This presents a problem in the safety of a subject.
As above described, in the case of conventional superconducting magnet apparatuses, it is difficult to enlarge openings, into each of which a subject is inserted, so as to prevent the subject from feeling claustrophobic, and to reduce magnetic field leakage, and to obtain a large high-magnetic-strength uniform magnetic field region. Moreover, it is also difficult to reduce the manufacturing costs to low values.
It is, accordingly, an object of the present invention to provide a superconducting magnet apparatus which deals with such problems of the conventional superconducting magnet apparatuses, and which enlarges openings for accommodating a subject to the extent that the subject does not feel claustrophobic and which has low magnetic field leakage, and which can realize a large high-magnetic-field-strength uniform magnetic field region and which reduces the manufacturing cost thereof to a low value, and to provide a magnetic resonance imaging system using this semiconducting magnet apparatus.